I would say that usually when you have a s (or x) before a vowel, you can make the liaison. This is in order to avoid hiatuses. Usually, it's intended to be easier to pronounce. But if you don't pronounce it, you will be understood anyway, don't worry for that.

Full official rules from l'Académie françaisehere. There are mandatory liaisons, forbidden ones, and optional cases (if you do the liaison then, it would be considered as “un parlé soutenu”, more or less the equivalent of the 'formal' register in English).